The best thing to do would be to bring it to a service center. When a phone gets water damage, the water gets under the components and oxidizes. First thing would be to clean the board with isopropyl alcohol, including under the metal shields, than check the entire board under microscope. Than remake all the solder joints that were destroyed by water and replace all the components that lost their pads. My personal opinion, it's not worthy a repair.
An image or video showing the defect would really help. Is it the backlight flickering or the image itself? Try looking on your screen in the sun to see if it is the backlight or the image itself. If it's the backlight, you need a new display. If it's the image, it could be a defective display, LVDS cable or graphic board.